Nanoparticles are of particular interest because of their use as catalysts, photocatalysts, adsorbents, sensors, ferrofluids, and due to their applications in optical, electronic, and magnetic devices. Since the characteristics imparted by the nanoparticles are dependant to a great part on the method of preparation, the synthesis of the nanoparticles can be critical.
Thiol-stabilized nanoparticles, in particular gold nanoparticles (Au-NPS), have been the focus of intense interest lately due to their potential use in the fields of optics, immunodiagnostics, and electronics. There exist a number of examples of small molecule stabilization using alkanethiols on a variety of metal species, including gold and palladium.
Recently, research has been directed to the application of synthetic (co)polymers as stabilization species for metal nanoparticles, such as stable gold colloids prepared by in-situ reduction (Mayer, A. B. R., Eur. Polym. Journal 1998, 34, 103-108); ATR grafting from polymerization to attach polymer chains to gold nanoparticles (Nu, S., et al. Angew. Chem. Int. Ed. 2001, 40, 4016-4018); gold nanoparticles decorated with covalently bound thiol-capped polystyrene macromolecules (Corbierre, M. K, et al. J. Am. Chem. Soc. 2001, 123, 10411-10412); and platinum nanoparticles with long perfluorinated carbon chains (Moreno-Mañas, M., et al. Chem. Commun. 2002, 60-61). However, these examples use processes of “grafting-to” and “grafting-from” for nanoparticle stabilization that utilize living polymerization techniques and suffer from disadvantages such as special reaction conditions, sensitivity to specific monomers, and necessitate expensive reagents and/or monomers.
Additional research in the area of nanoparticle synthesis has focused on the development of “pseudo-living” polymerization methods. Three principal approaches have been described to achieve this pseudo-living free-radical polymerization technique; reversible termination, reversible termination by ligand-transfer, and degenerative chain transfer. The first of these, typically referred to as nitroxide-mediated or stable free-radical polymerization (SFRP) has been exploited in the synthesis of controlled styrenic-based (co)polymers (Solomon, D. H., et al. U.S. Pat. No. 4,581,429; Goto, A.; et al. Macromolecules 1996, 29, 3050; Yoshida, E., et al. J. Polym. Sci., Part A: Polym. Chem. 1997, 35, 2371).
Subsequently, several groups disclosed an atom transfer radical polymerization (ATRP) process, which is a radical polymerization with reversible termination by ligand transfer to a metal complex. This technique has been shown to work especially well with styrenic and acrylate-based polymer monomers, similar to SFRP (Wang, J.-S., et al. Macromolecules 1995, 28, 7572; Sawamoto, M., et al., Trends Polym. Sci. 1996, 4, 371; Bandts, J. A. M., et al. J. Organomet. Chem. 1999, 584, 246).
Most recently, a third mechanism has been proposed for achieving “pseudo-living” polymerization character, which is a free-radical polymerization with reversible chain transfer (also termed degenerative chain transfer). It has been termed RAFT—reversible addition-fragmentation chain transfer polymerization (Le, T. P., et al., WO Patent 9801478; Chiefari, J., et al. Macromolecules 1998, 31, 5559-5562; Moad, G., et al. Polym. Int. 2000, 49, 993). This technique appears to offer several advantages over the previous ATRP and SFRP techniques, in that a vast array of monomers can be used, and the reaction can be performed under a broad range of experimental conditions using a variety of solvents.
Despite these recent advances, few methods have been described which allow for the covalent attachment of polymer chains directly to transition metal colloids or surfaces (the so-called grafting-to approach), as opposed to simple physical adsorption of a (co)polymer. Thus, there exists a need for a new, facile manner for the preparation of (co)polymer stabilized transition metal nanoparticles and surfaces. The approach described herein takes advantage of the method of polymer synthesis in which the polymers produced bear thiocarbonylthio end groups which can be reduced in situ in the presence of a transition metal sol or surface, yielding a polymer with a thiolate end-group which covalently bonds to the metal colloid or metal surface. This fast, facile, “one-step” synthetic procedure of simultaneous reduction of the polymer end-group and the metal colloid or surface in situ is significantly less demanding than the grafting-from approach which requires initial modification of the metal colloid with a suitable polymerization initiator, followed by subsequent polymerization. This new, rapid one-step method allows for the preparation of polymers suitable for covalent attachment on an industrial scale.